Cellular telephones may operate under a variety of standards including the code division multiple access (CDMA) cellular telephone communication. CDMA is a technique for spread-spectrum multiple-access digital communications that creates channels through the use of unique code sequences. It allows a number of mobile terminals to communicate with one or more base stations in the neighboring cell sites, simultaneously using the same frequency. In CDMA systems, signals can be received in the presence of high levels of narrow-band or wide-band interference. The practical limit of signal reception depends on the channel conditions and interference level. Types of interference include those generated when the signal is propagated through a multi-path channel, signals transmitted to and from other users in the same or other cell sites, as well as self-interference or noise generated at the device or mobile terminal. Typically, a receiver of the CDMA signal includes a number of demodulators or fingers, each of which can be used to demodulate signals received from a different propagation path. The demodulated signals from each finger may then be combined through maximum ratio combining or other similar combining algorithms. A Rake receiver is one of these types of demodulators. A separate finger can be used to search and identify signals arriving through various paths, usually with the help of a pilot signal.
A mobile station using the CDMA standard may constantly search a Pilot Channel of neighboring base stations and adjacent sectors of the serving base station, for a pilot that is sufficiently stronger than a pilot add threshold value, T_ADD which can be a static value or dynamic value as described in the standards. When such a pilot is found, the mobile station sends a Pilot Strength measurement message (PSMM) to the base station. As the mobile station moves from the region covered by one base station (sector) to another, the mobile station promotes certain pilots from the Neighbor Set to the Candidate Set, and notifies the base station or base stations of the promotion from the Neighbor Set to the Candidate Set via PSMM. The base station determines an Active Set according to the PSMM, and notifies the mobile station of the new Active Set via a Handoff Direction Message. The mobile station may maintain communication with all the base stations and base station sectors that are included in the active set. When the active set contains more than one base station, the mobile terminal is said to be in soft handoff with those base stations. When the active set contains more than one sector originating from the same base station, the mobile terminal is in softer handoff with those sectors. When the pilot of one of the base station (sectors) in the active set weakens to less than a pilot drop threshold value T_DROP for a time exceeding T_TDROP, the mobile station notifies the base station of the change through a PSMM. The base station may then determine and notify the mobile station of a new active set that will normally not include the base station or sector whose pilot was reported to have degraded below T_DROP for a duration of T_TDROP. Upon notification by the base station, the mobile station then demotes the weakened pilot to the Neighbor Set. This mechanism enables soft and softer handoff. The present invention involves an enhancement to the procedure for both cases of soft and softer handoff. The present invention can also be applied to other similar soft handoff process, such as that specified in 3GPP TR 25.922 (WCDMA standard), where the parameter “Time to Trigger” is similar to T_TDROP.